Samsung is working on solving some problems before updating, according to one network operator

Microsoft has invited 50 percent of eligible Windows Phone users to update to version 7.5, also known as "Mango," Microsoft's Eric Hautala said in a blog post on Monday.

The update process seems to be smoother that previous upgrades of Microsoft's smartphone operating system. Within the first hours of making it available, the company saw successful updates in every country, on every carrier, and for nearly every Windows Phone model, wrote Hautala, who is the general manager for Customer Experience Engineering.

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Users can follow the progress made by Microsoft and its operator partners on Microsoft's "Where's my phone update?" Web page.

Not all Windows Phone users are eligible for the update yet. Users of the HTC Surround and HD7S, LG Quantum, Samsung Focus, and Dell Venue Pro on AT&T in the U.S. will have to wait, as will owners of the Samsung Omnia 7 on SFR in France and on T-Mobile elsewhere in Europe.

Samsung is still working on some problems with the "Mango" update on the Omnia 7, said a spokeswoman for T-Mobile. The update will be distributed as soon as possible, maybe at the end of October, she said.

Samsung has a different take on why Windows Phone 7.5 as been delayed. The upgrade will be made available on the Omnia 7, but the "timing of the upgrade and its availability will depend on the mobile operators," a Samsung spokesman said.

Not all eligible users will be able to update at once. Windows Phones are selected to receive the update in a random order, "with absolutely no preference for carrier, model, or country," according to Hautala. Random sampling is an important engineering technique to accurately measure quality and pinpoint issues early, he said. "In some cases, this can have interesting consequences: A family, for example, can have identical Windows Phones, yet only one phone might get an update notification," Hautala writes.

When a user has received the update notification, the phone has to be connected to a PC or Mac to download and install Windows 7.5.